1. Field of the Invention
This invention relates to a filter cartridge with a porous filtering film. It more particularly relates to a filter cartridge for precise filtration to remove particles and microbes of the order of submicrons, specifically smaller than 10 .mu.m and preferably 1 .mu.m. Such a filter is used for instance in the filtration of chemicals and for the preparation of purified water or pure water for pharmaceutical manufacture in the pharmaceutical manufacturing industry, in the preparation of purified water or pure water used for producing extremely purified water which is used for precise work in the electronic industry such as for manufacturing semiconductor devices, in the filtration required in the manufacture of alcoholic beverages in the food industry, or in the preparation of purified water or pure water for ultrapure water in laboratories.
2. Background of the Invention
One example of a filter cartridge with porous filtering films which is extensively employed in the art is a pleat type filter cartridge. The conventional filter cartridge has a pleated porous filtering film and a cylindrical filtering film structure which is extended in parallel with the folds of the porous filtering film. The inner and outer cylindrical walls of the structure are protected by using plastic members. Other plastic members are sealingly bonded to the ends of the cylindrical filtering film structure.
In the pleat type filter cartridge thus constructed, the effective filtering area per unitary volume can be made large and therefore the filtration per unitary time can be also made large. Thus, the pleat type filter cartridge is extensively employed in the abovedescribed industries or in laboratories.
The filter cartridge when used is set in a predetermined housing. However, recently a disposable filter cartridge (called "a capsule type filter cartridge") has been used in which the effective filtering area is of the order of 0.05 to 0.4 m.sup.2 and the filtering film structure is integral with the housing. Such a filter has been used has been used in order to eliminate the operation of setting the filter cartridge in the housing or setting the filtering means on a filtering line in which small quantities of various solutions are filtered in short times with frequent change of filters. For the same purpose, a so-called "mini-filter-cartridge" is also used. The mini-filter-cartridge is used with a metal housing and has a small effective filtering area. The mini-filter-cartridge has a filtering part which is equal to or similar to that of the filtering film structure of the disposable filter cartridge and has a filtering film structure which uses a sealing part when combined with the housing.
The disposable filter cartridges and the mini filter cartridges are, in general, divided into two groups: a group of pleat type filter cartridges and a group of flat-plate-laminated filter cartridges. In the latter type, as disclosed by Japanese Patent Application (OPI) No. 129016/1981 (the term "OPI" as used herein means an "unexamined published application"), flat plate type filtering units are laminated.
The construction and the manufacturing methods of the disposable filter cartridges and the mini filter cartridges classified in the group of pleat type filter cartridges are fundamentally equal to those of the above-described pleat type filter cartridge which is used together with the housing. The manufacture of the pleat type disposable filter cartridges additionally includes a manufacturing step in which a filtering film structure formed is inserted into a plastic housing.
The construction and the manufacturing method of the flat-plate-laminated filter cartridges including the disposable filter cartridges and the mini filter cartridges are fundamentally as follows (see the aforementioned Japanese Patent Application (OPI) No. 129016/1981).
The flat plate type filtering unit, which is a fundamental component of the filter cartridge, is formed as follows. Two filtering films each having circular of polygonal inner or outer peripheries (the films being annular and coaxial when the inner and outer peripheries are circular) are bonded to a flat-plate-shaped supporting member having parallel surface at the inner and outer peripheries. The parallel surfaces may be formed by bonding two moldings together as the case may be. When necessary, there may be additional bonding between the filtering films and the supporting members at other predetermined points. In this connection, it is essential that the inner peripheral portions and the outer peripheral portions are respectively bonded together so that they are liquid tight.
The flat-plate-shaped supporting member is so designed that bonding the two filtering films to the two sides of the supporting member forms an internal space which serves as a path of a solution filtered. More specifically, the supporting member is of a frames structure supporting the two filtering films from inside and having an opening at the center to allow the filtered solution to flow to the central region of the filtering unit.
A filtering film structure is formed by stacking a plurality of flat-plate-type filtering units with bonding means provided at the centers of the filtering units. For protection of the filtering films, an upper protective plate is provided over the stack of filtering units with a predetermined space therebetween. For the same purpose, and, in the case of the mini filter cartridge, a lower protective plate is provided for the additional purpose of positively setting the housing. The filtering film structure thus formed is bonded to the housing in the case of manufacture of the disposable filter cartridge and it is coupled to a sealing member having a groove with which a sealing part such as an O-ring is engaged to set the cartridge in the housing.
What is required for these disposable or mini filter cartridges is, for instance, described as follows. They should have a large filtering area per unit volume, that is, their filtering modules should be compact. The filtration should be achieved with a small filtering pressure. They should have high filtration accuracy, and must be so designed as to be readily connected to or disconnected from the filtering line. Furthermore, they should be so designed that the porous filtering films are protected from damage when rubbed or subjected to shock during handling, and the quantity of solution remaining at the end of the filtration should be less.
However, in the conventional flat-plate type filtering unit, as disclosed for instance by Japanese Unexamined Patent Application Publication (OPI) No. 129016/1981, the flat-plate-shaped supporting member has a number of coaxial ribs and channels, and the filtering films are bonded to the ribs. Therefore, the parts of the filtering films bonded to the ribs cannot filter a solution. That is, the flat-plate type filtering unit suffers from difficulty that these parts are larger in area than the remaining part. That is, the effective filtering area is small.
In the filter in which the filtering films are bonded to the two surfaces of the flat-plate shaped supporting member at a number of points, the filtering films are liable to be strained with the result that it is difficulty to keep the outer and inner peripheral portions liquid-tight between the supporting member and the filtering films. In the case of a flat-plate type filtering unit which is compact, its manufacture is rather difficult in requiring a number of manufacturing steps with the result that it is high in manufacturing cost and is low in liquid tightness and accordingly in reliability. Furthermore in the case where the flat-plate-shaped supporting member is intricate in structure as in that disclosed by the above-described Japanese Patent Application in which the path of a solution is defined by a number of ribs and channels, the filtration resistance is so high that the solution cannot smoothly flow and the removal of the residual air or solution cannot be achieved with ease. In addition, the supporting member is rather high both in manufacturing cost and in assembling cost.
As was described above, the flat-plate-shaped supporting member has a number of coaxial ribs and channels, and the filtering films are bonded to the ribs. Therefore, when pressure is applied to the filtering unit from its secondary side of filtration for back washing or other purposes, the filtering films will be expanded towards the primary side of filtration, and would be stretched up if stretchable. At worst, they may become detached from the supporting member.
If, in the case where the central portions of flat-plate type filtering units are bonded to one another to stack the filtering units, the filtering units are inclined with respect to one another, then they may be brought into contact with one another. This difficulty may be overcome by increasing the distance of lamination of the filtering units. However, the method raises another problem that the volume of the resultant filter cartridge is increased.